1. FIELD OF THE INVENTION
The invention relates to apparatus for and methods of producing a hot asphaltic material and also and particularly to apparatus for and methods of producing a hot asphaltic material by combining recycled asphaltic pavement with virgin aggregate.
2. DESCRIPTION OF RELATED ART
In accordance with current environmental protection efforts, demands are made on the highway construction and repair industry to use equipment which complies with environmental requirements. At the same time the equipment needs to be capable of producing quality highway paving materials in an efficient manner. In one approach to conservation and efficiency, recycled asphalt pavement ("RAP") is combined with virgin aggregate material ("VAM") and mixed with the addition of hot liquid asphalt cement. The mixing of the constituent materials typically takes place at elevated temperatures, and a propensity for problems appears to be present in that at a low temperature the asphalt cement thickens and hardens. Thus too low a temperature may not allow for adequate mixing of the constituents and higher temperatures can produce unwanted levels of smoke emissions from some types of RAP and asphalt binders as the asphaltic constituents begin to oxidize and to vaporize.
VAM, the virgin aggregate, is readily dried in a continuous drying process using drum driers. The driers feature large drums which rotate about axes disposed typically at a slight incline from the horizontal and which are typically equipped with an open flame burner and a blower at one end. The inside surfaces of such drums are further equipped with spaced sets of flights which lift the material and dump it in a falling curtain of the particulate material which exposes it to the hot gases emanating from the burner.
Asphaltic materials are produced in both continuous processes and in batch processes. In continuous mixing processes, drums such as those used for the described drum driers are used as mixing drums, of course with proper modifications for (he addition of other materials and the asphalt cement. Continuous processes, while having been used with some variations, typically need to deal with the volatility of the asphalt materials. RAP, the recycled material, tends to smoke when subjected to open flames or excessive heat. RAP can, however, be introduced into less hot regions of drying and mixing drums. According to a known procedure, the VAM is superheated to temperatures well above a desirable temperature range for the final asphaltic product. The ultimate mixture then yields an acceptable average temperature after the addition of the relatively cooler RAP. Yet, the addition of asphalt cement in contact with the still superheated VAM can also lead to the generation of unwanted "blue" smoke, an organic asphalt gas which is undesirable from an environmental standpoint.
According to one prior art apparatus for mixing VAM and asphalt cement the disclosed structure divides the drum into a drying section and into a mixing section. A burner feeds hot gases directly into the drying section of the drum, while the mixing section is partially shielded from the direct contact with the flame of the burner. The apparatus also provides a looped gas return which permits some gas from the mixing chamber to be fed via a return duct to the burner. The prior art apparatus nevertheless provides for gas to be released from the mixing chamber to the atmosphere. In a steady state operation, such release would also occur continuously, as hot gases are continuously being generated by the burner.
In another continuous process, instead of using drums for mixing, another type of apparatus known as a "pugmill" is used. The constituents of the mix are fed into storage hoppers, and are continuously dispensed in specified proportions into a drier. The preheated virgin aggregates may be superheated and are usable in such a system for some preheating of RAP to occur by heat transfer from the VAM with an associated cooling of the superheated VAM. The liquid asphalt cement is then added directly to the dry mix within the pugmill. The output from the pugmill can then be discharged directly into a truck for immediate use on a paving job. In the alternative, the output from the pugmill may be transferred to a silo from where it would be loaded into trucks to be carried to a job site.
When pugmills are used in continuous mixing operations, the materials are sometimes dry-mixed at one end of the pugmill and are thereafter coated with the liquid asphalt cement and wet-mixed in a down-stream portion of the pugmill before being discharged at the other end of the pugmill.
In a prior art batch-type mixing operation which used both VAM and RAP as constituents for the asphaltic material, the VAM was superheated, approximately to a temperature of 500 degrees Fahrenheit, and transferred to a weigh hopper to be dispensed in batches into the pugmill for final mixing and discharge into trucks. The RAP was metered in desirable proportions into the weigh hopper to become intermixed with the VAM or portions thereof in the weigh hopper.
One of the disadvantages of the described system is an inability to control the temperatures of the mix. Another disadvantage is the heat loss from the stored aggregate, which in turn is translated to inefficiency and to an inability to tightly control temperatures of the final mix. The final product may consequently experience the symptoms discussed above that are observed when mixing occurs at a temperature which is either too low or too high.
Another known disadvantage of prior art systems is what is known as a venting problem that occurs when comparatively cool RAP is combined suddenly with super-heated VAM, as the aggregates drop out of a weigh hopper into a pugmill. Since the Rap frequently contains a significant amount moisture, a rapid generation of steam has frequently caused such a venting problem. The generated steam is not readily vented and generates momentarily high pressures, causing reactions similar to small explosions.
The known prior art also does not provide for an efficient apparatus and method in which a pugmill mixes specified portions of RAP and VAM in a continuous mixing process, and particularly not one in which such apparatus has the compactness to permit ready portability as is desirable for many highway construction projects.